Final answer:
The suffix '-pnea' refers to breathing, as seen in terms like 'apnea' (cessation of breathing) and 'dyspnea' (difficult breathing).
Step-by-step explanation:
The suffix that means "breathing" is -pnea. This can be seen in medical terms like "apnea", which refers to a temporary cessation of breathing, and "dyspnea", which means difficult or labored breathing. The other suffixes represent different medical concepts: "-capnia" refers to the level of carbon dioxide in the blood, "-oxea" is not a standard medical suffix, and "-phonia" pertains to the sounds produced by the voice, as in "dysphonia", which means difficulty in speaking due to a voice impairment.